This proposal describes studies that are designed to determine fine structural features of relaxin producing cells in endometrial glands of the guinea pig. Specific projects include morphologic and immunocytochemical studies of endometrial gland cells in uteri from the following: 1) cycling, pregnant and lactating animals, 2) ovariectomized animals that have been induced to produce relaxin by injections of estradiol and progesterone, and 3) pregnant animals and overiectomized animals induced to produce relaxin treated with agents that are known to precipitate parturition, e.g., oxytocin and prostaglandin.